I figured this process out as I went along, so there aren't any progress pictures, but I promise it's easier than it sounds. Pictured below: a tiny VW bus parked on my new "poured cement" driveway.
You Will Need:
Illustration board (a.k.a. mat board - use white, ivory, or gray, not black)
Paintbrush (I used a 3/4" wide artists' brush - anything smaller and it would have taken a lot longer)
And of key importance...SAND! The finer, the better. (I live fairly close to the ocean, so I used beach sand; if you are hopelessly landlocked, toy stores that sell children's sandboxes usually sell bags of sand, too.)
Cut your illustration board to the size and shape you want. Fit it into place to make sure the sizing is right.
Coat the surface of your illustration board with a thin, even coat of white glue (use a brush). Sprinkle the sand onto the board, covering ALL of the glue. Allow to dry, and gently tap off any excess sand.
If any spots on the board didn't pick up the sand, apply a dab of glue and sprinkling of sand to the affected areas. Yes, this will create a slightly uneven surface. However, it looks exactly like old, cracked-but-repaired/unevenly poured cement if you do this. So, for a newer-looking driveway, be extra careful to cover everything in one application. For an old, repaired sidewalk, don't worry too much about it.
Dilute gesso and gray paint with water, and brush onto the sand. The color of gray paint that works best depends on what color the sand is. The sand I used was a nice medium grayish tan with black and white flecks, so a lighter, cooler gray was called for (I used Martha Stewart's craft paint in Gray Wolf). If you are using white (or light tan) sand, you may want to omit the gesso. Allow to dry, and if you are happy with the color (which will be lighter when dry), brush on a second coat. (It's a good idea to test this on small pieces of board until you get the color just right.)
Pictured below: cement walkway to concrete front porch.
Once the paint is good and dry (and you are happy with the color), seal your cement. I used two light coats of hard-finish Mod Podge.
Finally, when the sealer is dry, use the knife to score the cement where desired (you might want to change the blade when you're done). Scoring can imitate lines from cement being poured in small areas (such as with old sidewalks), or score unevenly to create cracks. If you wish, you can add dabs of paint to simulate oil drips, rust from an old car, decaying wads of gum, or moss growing in cracks.
I initially experimented with first sealing, then scoring, then painting the board (basing this process on Otterine's concrete tutorial). I quickly found it wasn't necessary, since the sand obscures the scoring and any base coat of paint. It's best to apply the sand first.
Close-up of the VW bus parked on the driveway. (I made the vanity plate by using a license plate generator, choosing the brown/yellow color scheme that was used in the 1960s rather than the white-red-blue California plates in use now. Then I took a screenshot and scaled it down.)